1. Field of the Invention
The present invention is directed to an information recording medium suitable for use in various types of imaging recording files and large capacity memory means including video disks and digital audio disks. It relates in particular to the so-called direct read after write (DRAW) type information recording medium capable of recording and reproducing information.
2. Description of the Prior Art
Various DRAW type information recording media have been proposed. One example of such a recording medium uses laser beams modified by recording information signals which are irradiated on a thin membrane of a low melting metal formed on a substrate to melt or evaporate the metal membrane under the concentrated heating in accordance with the information contained therein, thereby forming apertures or recording bits in the metal membrane, so that the recording is carried out as a change in the configuration of the medium. However, formation of the recording bits in this way requires a large beam power for writing and in addition involves a difficulty in controlling the shape of the recording bits resulting from melting to increase the noise level and lower the image resolution performance, thus making it difficult to obtain high density recording.
There has also been suggested a recording medium which utilizes a change resulting in the optical characteristics instead of a change in the configuration of the recording layer for writing and reading purposes. For example, in the medium disclosed in Japanese Patent Laid-Open No. 138145/1977, recording is performed in the form of local crystallization in an amorphous recording layer caused by the heating of recording laser beams and the recorded information is read out using the difference in optical characteristics between the crystallized and the non-crystallized regions. However, the recording due to the crystallization of the amorphous recording layer had to be done under a condition of gradual heating and gradual cooling to cause crystallization. In the case of a disk-like information recording medium, for example, of 300 mm diameter and using signals of about 5 MHz and rotating the disk on the order of 1800 rpm, this corresponds to a recording condition of rapid heating and rapid cooling. Accordingly, it is difficult to achieve any of the desired crystallization. On the other hand, those materials which are capable of crystallizing under such a high speed recording condition are unstable and are not suitable for storage for a long time. Particularly, in the case where the recording layer of the amorphous material is formed by vapor deposition, the material which has been formed into such an amorphous layer upon vapor deposition does not go through the step of atom rearrangement for crystallization and the desired stable state of the recording layer in the amorphous form is that of the crystalized form. Consequently, the amorphous phase in the non-recording region or in the not yet recorded region is gradually crystallized during long-time storage to cause lack of stability and a reduction in the signal-to-noise ratio. Further, since there is no substantial change obtained in the optical characteristics merely by the crystallization of the amorphous layer, it is impossible to obtain high resolution, a high S/N ratio, and a high recording density.
A further example of recording medium is disclosed in Japanese Patent Laid-Open No. 22095/1982 wherein the recording layer comprises a first layer composed of Au or Ag and a second layer composed of In or Sn, and solid phase diffusion is caused between the first and second layers by heating with the irradiation of recording laser beams. The recorded information is read out due to the change in the reflectance between the recording region where the solid phase diffusion is caused and the other region where no such solid phase diffusion exists. However, since the reflectance is mainly determined by the material of the second layer disposed on the side where the reading laser beams are entered in this mode of recording, an insufficiently large change can be obtained upon reading out the recorded information. Therefore, there is no high resolution, an insufficiently high S/N ratio, and an insufficiently high recording density. Since the solid phase diffusion is dependent on temperature and time, the solid phase diffusion proceeds gradually in the non-recorded region or not yet recorded region during a long time storage of the recording medium and thereby worsens the S/N ratio.
In a still further example of a recording medium, there is disclosed in Japanese Patent Laid-Open No. 18031/1982 a dual layer structure composed of a high reflection membrane having a higher reflectance to reading beams and a low reflection membrane having a lower reflectance and a higher absorbance to writing beams on a transparent substrate. Recording laser beams are irradiated upon writing on the side of the lower reflection membrane to cause alloying or mixing between both of the metals in the high reflection membrane and the low reflection membrane by heating of the laser beams. Information is recorded by denaturing the high reflection membrane due to alloying or mixing and thereby reducing the reflectance. The information is then read out by irradiating the reading beams on the side of the high reflection membrane and utilizing the difference in reflectance between the low reflectance region previously written and denatured and the other regions. However, since the recording and the reading laser beams have to be irradiated on opposite sides in this recording medium, there results the disadvantage of an increase in the size of the recording and reproducing apparatus and a complicated handling for the recording medium.
It is generally required that recording laser beams and, in some instances, reading laser beams, be condensed or focused on the recording layer, a light absorbing layer for use in heating the recording layer, or the reflection layer. In this case, it is desirable that the laser beams be irradiated on the side of the substrate having a larger thickness, for example, about 1 mm on which the above-mentioned layers are deposited in order to avoid as much as possible undesired effects resulting in spots in the laser beam focusing area such as are caused by minute scratches, shadows of dusts and scattering on the surface of the information recording medium. If the laser beams are irradiated on the side of the substrate, a certain distance can be obtained between the area where the laser beams are focused and the surface of the substrate, that is, the surface of the medium where such scratches or dusts are present. The surface is kept out of the focal depth of the focusing lens system and, accordingly, reduces the effects of the scratches and the dusts on the focusing area. However, it is generally impossible to use such a mode of recording and reproduction in an information recording medium of the conventional DRAW type.
While it is preferred to use a transparent substrate for the information recording medium, for example, a plastic substrate, from the point of view of economy and also for ease of handleability, deformation of the substrate is inevitable in a large diameter disk due to the residual inner stresses resulting from molding of the substrate, and swelling due to humidity or the like which gives a significant undesired effect on the energy distribution of the focused laser beams and provides a great deal of trouble in recording and reproducing the information. To prevent the deformation of the substrate, it is extremely effective to sandwich a recording layer between a pair of opposed substrates made of identical materials and in the same configuration. However, in the recording medium of the foregoing type for recording information depending on a bit-producing change in the configuration of the medium through melting and evaporation of the metal layer, the substrate of the sandwich structure cannot be used because it requires a space for permitting the configurational change, that is, a space for accommodating the bulge of the metal at the circumferential edge of bits caused by the surface tension accompanying bit formation.